1. Field of the Invention
The invention relates to a process for the ring-chlorination of aromatic hydrocarbons in the liquid phase in the presence of Friedel-Crafts catalysts and in the presence of co-catalysts.
2. Description of the Related Art
The reaction of aromatic hydrocarbons, such as toluene, with gaseous chlorine in the liquid phase to give ring-substituted chlorine derivatives, such as monochlorotoluene, is known (Ullmanns Enzyklopadie der Technischen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry), 4th edition, Volume 9, page 499 et seq.). This chlorination is in general carried out in the presence of Friedel-Crafts catalysts, such as iron(III) chloride, antimony chlorides or aluminium chloride. The resulting chlorination product is usually a mixture of isomeric monochlorinated and polychlorinated compounds. If FeCl.sub.3 is used, a mixture of monochlorotoluenes and dichlorotoluenes is obtained, for example, from toluene; in the monochlorotoluene fraction, the main product is o-chlorotoluene, alongside p-chlorotoluene and a small amount of m-chlorotoluene.
Since p-chloroalkylbenzenes, such as p-chlorotoluene, in particular are useful intermediate products, there has been no lack of attempts in the past to guide the chlorination so that the ratio of o- to p-chloroalkylbenzenes is reduced, that is to say attempts have been made to discover conditions which promote the formation of p-chloroalkylbenzenes.
It is known from U.S. Pat. No. 3,226,447 that an o/p ratio of 1.2 can be obtained in the chlorination of toluene by addition of sulphur compounds containing divalent sulphur to the Friedel-Crafts catalyst. The disadvantage of this process is the fact that this ratio, which is still not very favourable, is achieved only when antimony salts are used as the Friedel-Crafts catalysts. A further disadvantage is that according to Example 16 in that specification, the amounts of catalyst components required are very high, and in particular 1% by weight for each of the two catalytic additives. As the o/p ratio with a value of &gt;1 shows, more o- than p-chlorotoluene is still formed in this process.
The chlorination of toluene with, for example FeCl.sub.3 and S.sub.2 Cl.sub.2 is likewise described in German Offenlegungsschrift 1,543,020 and U.S. Pat. No. 4,031,144. The resulting ratio of o/p=1.03-1.10 is still unsatisfactorily high.
The chlorination of toluene with Friedel-Crafts catalysts using thianthrenes or substituted thianthrenes is described in U.S. Pat. No. 4,031,147, U.S. Pat. No. 4,069,263, U.S. Pat. No. 4,069,264 and U.S. Pat. No. 4,250,122. The most favourable o/p ratios which can be achieved are about 0.7, but are obtained either only by using antimony salts or, in the case of the use of iron salts, only at very low reaction temperatures of about 0.degree. C. Both these cases are decidedly unfavourable industrially. The co-catalytic action of thianthrenes is thus greatly impeded by traces of iron when antimony salts are used, and can be realized only with difficulty in industry. Furthermore, the reaction is so highly exothermic that removal of the heat at about 0.degree. C. by cooling with brine becomes very expensive. The thianthrenes are furthermore already destroyed from ubiquitous traces of water under customary reaction conditions and thus lose their activity.
The chlorination of toluene in the presence of Lewis acids and phenoxathiines is furthermore known from U.S. Pat. No. 4,289,916, European Patent Specification 63,384 and European Patent Specification 173,222. The o/p ratio of 0.6 which can be achieved according to Example 1 of European Patent Specification 173,222 is again achieved only by the industrially extremely unfavourable use of antimony chloride and the high amount of 0.29% by weight of co-catalyst. If FeCl.sub.3 is used instead of antimony chloride, an o/p ratio of 0.68 is obtained, but again only at the industrially extremely unfavourable low reaction temperature of 5.degree. C. At an industrially advantageous reaction temperature of 50.degree. C., the o/p ratio increases in the presence of FeCl.sub.3 and the phenoxathiine derivative claimed in European Patent Specification 173,222 to 0.88; this can be seen from Comparison Example No. 50 from European Patent Specification 292,824. In the above U.S. Pat. No. 4,289,916 and European Patent Specification 63,384, a most favourable o/p ratio of about 0.8 is described. Here also, the o/p ratio can be lowered to 0.65 if antimony chlorides and a reaction temperature of 20.degree. C., that is to say industrially unfavourable conditions, are used instead of FeCl.sub.3. Phenoxathiines are also destroyed in the presence of traces of water.
Chlorination of toluene in the presence of Friedel-Crafts catalysts and N-substituted phenothiazines is known from European Patent Specification 126,669. The o/p ratio of 0.84 is also adversely high here.
The chlorination of toluene in the presence of certain zeolites is known from European Patent Specification 112,722, European Patent Specification 154,236 and European Patent Specification 248,931, an o/p ratio of about 0.3 being achieved with the addition of, for example, halogenocarboxylic acid halides as moderators. The considerable amounts of 5% by weight of zeolite and 1% by weight of moderators are a disadvantage of this process. As our own experiments have shown, this result must be paid for with the considerable disadvantage that very large amounts (up to 8% by weight) of benzyl chlorides occur in the resulting mixtures. However, the formation of benzyl chlorides interferes to a quite exceptional degree in the subsequent customary working up distillation.
The chlorination of alkylbenzenes having up to 12C atoms in the side chain in the presence of Friedel-Crafts catalysts and thiazepine derivatives as co-catalysts is known from European Patent Specification 292,824. A characteristic feature of the structure of the co-catalysts claimed in that specification is that 3 single bonds always originate from the nitrogen atom in the heterocyclic 7-membered ring. According to European Patent Specification 292,824, this N atom cannot enter into a double bond. The o/p ratio which can be achieved for toluene by this process is 0.64 in the best case (see Example 49 of European Patent Specification 292,824).
A process for the chlorination of alkylbenzenes having up to 12C atoms in the side chain in the presence of Friedel-Crafts catalysts and of thiazocin derivatives as co-catalysts is furthermore known from German Offenlegungsschrift 3,815,537 and German Offenlegungsschrift 3,824,068. Here also, a characteristic feature of the active co-catalyst structure is that only single bonds and no double bonds originate from the nitrogen atom of the heterocyclic 8-membered ring. The o/p ratio which can be achieved for toluene by this process is in the best case 0.78 (see Example 14 of German Offenlegungsschrift 3,815,537 or German Offenlegungsschrift 3,824,068).